Method and apparatus for providing symmetrical output data for a double data rate dram

ABSTRACT

An apparatus and method is disclosed to compensate for skew and asymmetry of a locally processed system clock used to synchronize an output signal, e.g., a data signal or a timing signal, from a logic circuit, for example a memory device. A first phase detector, array of delay lock loop (DLL) delay elements and accompanying circuitry are disclosed to phase-lock the rising edge of the output signal with the rising edge of the system clock XCLK signal. Additionally, a comparator circuit, a register delay, an array of DLL delay elements and accompanying circuitry are disclosed to add or subtract delay from the falling edge of the DQ signal in order to produce a symmetrical output of the DQ signal.

FIELD OF THE INVENTION

The present invention relates generally to synchronizing the timing ofdata transfer with a system clock using a delay lock loop circuit. Moreparticularly, the present invention relates to a method and apparatusfor producing a symmetrical data clock by adding to or subtractingcompensating delays to the falling edge of an internal clock.

BACKGROUND OF THE INVENTION

Modern high-speed integrated circuit devices, such as synchronousdynamic random access memories (SDRAM), microprocessors, etc., rely uponclock signals to control the flow of commands, data, addresses, etc.,into, through, and out of the devices. Additionally, new types ofcircuit architectures such as SLDRAM require individual circuits to workin unison even though such circuits may individually operate atdifferent speeds. As a result, the ability to synchronize the operationof a circuit through the generation of local clock signals has becomeincreasingly more important. Conventionally, data transfer operationsare initiated at the edges of the local clock signals (i.e., transitionsfrom high to low or low to high).

In synchronous systems, integrated circuits are synchronized to a commonreference system clock. This synchronization often cannot be achievedsimply by distributing a single system clock to each of the integratedcircuits for the following reason, among others. When an integratedcircuit receives a system clock, the circuit often must condition thesystem clock before the circuit can use the clock. For example, thecircuit may buffer the incoming system clock or may convert the incomingsystem clock from one voltage level to another. This processingintroduces its own delay and/or skew, with the result that the locallyprocessed system clock, often will no longer be adequately synchronizedwith the incoming system clock. In addition, the system clock itself mayhave a certain amount of skew within a tolerance set by systemspecifications. For example, an exemplary DDR SDRAM system may allow asystem clock skewed to have a duty cycle of 55%/45%. The trend towardsfaster system clock speeds further aggravates this problem since fasterclock speeds reduce the amount of delay, or clock skew, which can betolerated.

To remedy this problem, an additional circuit is conventionally used tosynchronize the locally processed clock to the system clock. Two commoncircuits which are used for this purpose are the phase-locked loop (PLL)and the delay-locked loop (DLL). In the phase-locked loop (PLL), avoltage-controlled oscillator produces the local clock. The phases ofthe local clock and the system clock are compared by a phase-frequencydetector, with the resulting error signal used to drive thevoltage-controlled oscillator via a loop filter. The feedback via theloop filter phase locks the local clock to the system clock.

In contrast, the delay-locked loop (DLL) generates a synchronized localclock by delaying the incoming system clock by an integer number ofperiods. More specifically, the buffers, voltage level converters, etc.of the integrated circuit device, for example the input buffers of anSDRAM memory device, introduce a certain amount of delay. Thedelay-locked loop (DLL) then introduces an additional amount of delaysuch that the resulting local clock is synchronous with the incomingsystem clock.

In certain synchronous circuit devices, for example double data rate(DDR) dynamic random access memory (DRAM), wherein operations areinitiated on both the rising and the falling edges of the clock signals,it is known to employ a delay lock loop (DLL) to synchronize the outputdata with the system clock (XCLK) using a phase detector. In anexemplary case, the transition of the data signal is perfectly alignedwith the rising or falling edge of the XCLK. The time from the rising orfalling edge of the data clock to the time when the data is available onthe output data bus (tAC) is within specifications. A phase detector isconventionally used to lock the rising edge of the output data signalfrom the DLL (DQ) to the rising edge of the XCLK. Since the rising edgeof the DQ signal is phase-locked to the rising edge of the XCLK signal,the rising edge of data being output from the device is synchronizedwith the system clock XCLK.

FIG. 1 depicts a DDR DRAM data synchronizing circuit using a DLL as ispresently contemplated in the art. A DQ data output signal from an arrayis input to output buffer 23 and has its timing adjusted to besynchronized with the XCLK signal 8. At system initialization, a phasedetector 2 is activated by an initialization signal 4. The phasedetector 2 compares the phase of the CLKIN signal 6, a processed signalderived from the XCLK signal 8, with the OUT_MDL signal 10, a model ofthe data output signal DQ. The phase detector 2 then adjusts the DLLdelay elements 12 using respective ShiftR 14 and ShiftL 16 signals, torespectively decrease or increase the time delay added to the CLKINsignal 6 with respect to the OUT_MDL signal 10.

The Output Buffer Model 19 models the delays generated by the OutputBuffer 23 and the CLK Buffer Model 21 models the delays generated by theInput Buffer 7 to produce an OUT_MDL signal 10 such that alignment ofthe OUT_MDL signal 10 with the CLKIN signal 6 will result in alignmentof the XCLK signal 8 with the DQ data output signal 24. By adjusting thedelay of the CLKIN signal 6 through the DLL delay elements 12, the phasedetector 2 can align the rising edge of the DQ output signal 24 with therising edge of the XCLK signal 8.

The output data signal DQ 24 is provided to a data pad 31 and issynchronized with the system clock XCLK 8.

In addition, the FIG. 1 circuit can also be used to adjust an outputtoggle clock signal DQS as shown in FIG. 9. In this case, an additionaloutput buffer 23 a is used to generate the DQS signal at pad 31 a. TheDQS signal can be used for timing purposes, such as a data strobesignal. For purposes of simplifying the discussion below, the backgrounddiscussion and the discussion of the invention will be described in thecontext of synchronizing the data output signal DQ with the system clockXCLK 8, but the discussions herein apply to also synchronizing a DQSsignal with the system clock XCLK.

FIG. 2 is a timing diagram for the synchronizing circuitry of FIG. 1. Asshown in FIG. 2, the rising edge 26 of the XCLK signal 9, which iscarried on the XCLK signal line 8 of FIG. 1, is aligned with the risingedge 28 of the DQ signal 25, which is carried on the DQ signal line 24of FIG. 1. As is indicated by the arrows shown in FIG. 2, the risingedge 30 of the DLLCLK signal 33 (carried on the DLLCLK signal line 32 ofFIG. 1) initiates the rise and fall of the DLLR signal 21 (carried onthe DLLR signal line 20 of FIG. 1), through the Rise Fall CLK Generator18 (FIG. 1), which in turn initiates the rising edge 28 of the DQ signal25. Likewise, the rising edge 34 of the DLLCLK* signal 37 (carried onthe DLLCLK* signal line 36) initiates the rise and fall of the DLLFsignal 23 (carried on the DLLF signal line 22 of FIG. 1) which in turninitiates the falling edge 42 of the DQ signal 25. For proper datasynchronization, the rising edges of the XCLK 9 and DQ 25 should bealigned within an allowed tolerance and the duty cycle of the dataoutput timing signal DQ 25 should be within the specifications for thesystem in which the synchronizing circuitry will be used.

Unfortunately, however, not all synchronizing circuitry components areideal or even exemplary. Non-symmetrical delays can be created by theinput processing of the system clock including input buffering of thesystem clock signal using the buffer 7. The system clock itself mayexhibit an asymmetric duty cycle, for example, up to a 55/45 duty cyclefor a typical SDRAM. Variations in layout, fabrication processes,operating temperatures and voltages, and the like, result innon-symmetrical delays among the DLL Delay Elements 12. All of thesenon-symmetrical delays produce output timing signals of the DLLexhibiting a difference between the duration of a high (tPHL) and low(tPLH) portion of the DLL output signal. As shown in FIG. 6, the highand low tPHL and tPLH signal portions, respectively, refer to the amountof time between transitions of the signal. If a signal remains high fora period longer than it stays low, then that signal is said to beasymmetric. On the other hand, if a signal is high and low for equalperiods of time, then that signal is said to be symmetric.

Non-symmetrical delays also result in a skewed data eye and a largerdifference 46 (FIG. 2) between the falling edge 44 of the XCLK signal 9and the falling edge 42 of the DQ signal 25. In other words, as shown inFIG. 2, for an XCLK signal 9 having a 55/45 duty cycle, due toinconsistencies in the DLL delay elements 12 (FIG. 1), the DLLCLK 33 andDLLCLK* 37 signals may have a duty cycle of 40/60. Because it is therising edge 30 of the DLLCLK signal 33 and the rising edge 34 of theDLLCLK* signal 37 from which the rising 28 and falling 42 edges,respectively, of the DQ signal 25 result, the non-symmetrical delays mayresult in a non-functional system. Furthermore, because the number ofDLL Delay Elements used is cycle time dependent, the skew and difference46 are also cycle time dependent. This unpredictable skew is undesirablefor reliable high speed performance.

Therefore, there is a strong desire and need for synchronizing circuitrywhich compensates for the lack of symmetry in a signal synchronized by adelay-locked loop circuit with a system clock, thus enabling morereliable performance at high speeds.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus to compensate forskew and asymmetry of a locally processed system clock used tosynchronize an output signal (e.g., a DQ data or DQS timing outputsignal) from a digital circuit, for example a memory device.

In its apparatus aspects the invention provides a first phase detector,an array of DLL delay elements and accompanying circuitry to phase-lockthe rising edge of an output signal (e.g., DQ or DQS signal) with therising edge of the system clock XCLK signal. Additionally, a comparatorcircuit, a register delay, an array of DLL delay elements andaccompanying circuitry are provided to add or subtract delay from thefalling edge of the output signal in order to produce a symmetricaloutput signal. The symmetrical output signal provides an improved timingmargin for a given cycle time.

In its method aspects, the invention compares a processed system clockwith a signal representative of an output signal (e.g., DQ or DQSsignal) to adjust a setting of a delay circuit to phase-lock a risingedge of the output signal to a rising edge of an unprocessed systemclock signal, producing a first delayed timing signal. A second delaycircuit is adjusted according to asymmetries in a duty cycle of thefirst delayed timing signal, producing at least a second delayed timingsignal. At least the first and second delayed timing signals are used toproduce a substantially symmetrical output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the invention will be moreclearly understood from the following detailed description which isprovided in connection with the accompanying drawings in which:

FIG. 1 illustrates a block diagram of a portion of a conventionalcircuit for generating a synchronizing data output signal;

FIG. 2 illustrates a timing diagram for selected signals of FIG. 1;

FIG. 3 illustrates a block diagram of a portion of a circuit forgenerating a synchronizing data output signal in accordance with thepresent invention;

FIG. 4 illustrates a diagram of a portion of the circuit of FIG. 3;

FIG. 5 illustrates a block diagram of another portion of the circuit ofFIG. 3;

FIG. 6 illustrates a timing diagram for selected signals of FIG. 3;

FIG. 7 illustrates a processor system employing a method and apparatusof the present invention;

FIG. 8 illustrates a partial block diagram of a memory systemconstructed in accordance with an embodiment of the invention; and

FIG. 9 illustrates a variation of the FIG. 1 circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For simplification, the invention will now be described with referenceto synchronization of data output (DQ) from a memory device, it beingunderstood that a memory device is not required, and that the inventionapplies to synchronizing the data output of any digital circuit whichoutputs data in a synchronized manner with reference to a system clock.In addition, the invention can also be used to produce a timing outputsignal DQS which is synchronized with a system clock.

FIG. 3 is a block diagram of an embodiment of a data synchronizingcircuit according to the present invention. The embodiment in FIG. 3includes a first phase detector 108 which, like phase detector 2 of FIG.1, detects the relative phase between the CLKIN signal 103, a derivativeof the system clock signal XCLK 102, and the OUT_MDL signal 126, whichmodels the timing of the output buffer 134 which buffers andsynchronizes the data output DQ signal 138. In response to a phasedifference between the CLKIN signal 103 and the OUT_MDL signal 126, thefirst phase detector 108 adjusts the delay of DLL Delay Elements 106 tothe CLKIN signal 103 by sending respective ShiftL 110 and ShiftR 112signals to the DLL Delay Elements 106 to phase-lock the rising edges ofthe CLKIN 103 and OUT_MDL 126 signals. The Output Buffer Model 130models the delays generated by the Output Buffer 134 and the CLK BufferModel 128 models the delays generated by the Input Buffer 104 to producean OUT_MDL signal 126 such that alignment of the OUT_MDL signal 126 withthe CLKIN signal 103 will result in alignment of the XCLK signal 102with the DQ signal 138. Phase-locking the rising edges of the CLKIN 103and OUT_MDL 126 signals respectively causes the rising edges of the XCLK602 and DQ 624 signals (FIG. 6) to align.

Once the first phase detector 108 has achieved a phase-lock, it outputsa phase-lock signal 124 to initiate operation of the comparator 148. Thecomparator 148 compares the relative time durations of the high tPLH andlow tPHL portions of the DLLCLK signal 118 and the DLLCLK* signal 122,which is an inverted DLLCLK signal. In response to durationaldifferences between tPLH and tPHL, the comparator 148 generates add andsubtract signals 144, 146. The add and subtract signals 144, 146 areused in the Rise Fall CLK Generator 132 to control the amount of delayadded to or subtracted from the DLLCLK* signal 122 prior to generationof the DLLF signal 142. The DLLR and DLLF signals 140, 142 are generatedin the Rise Fall CLK Generator 132 to correspond to the rising edge ofthe DLLCLK and (delayed) DLLCLK* signals 118, 122, respectively, and areused in the Output Buffer 134 to produce the output data timing signal138. As noted, the DLLR and DLLF signals 140, 142 are also used in theOutput Buffer Model and CLK Buffer Model blocks 130, 128 to produce theOUT_MDL signal 126. The output data signal DQ on line 138 has both itsrising and falling edges synchronized with the system clock XCLK 102.

FIG. 4 illustrates an exemplary embodiment of circuitry within thecomparator 148. A first converter circuit 211 is connected between areference voltage Vref and ground and includes two serially connectedenabling transistors 202 and 204 and pull-down transistors 206.Transistor 202 is connected to Vref while transistor 206 is connected toground. When transistor 202 is on, a capacitor 214 is connected betweenthe reference voltage Vref and ground as shown in FIG. 4. The upperplate of the capacitor, connected to the reference voltage Vref, is alsoconnected to a first input (+) of a comparison circuit 220. The gates ofthe enabling transistors 202 and 204 are controlled by the phase locksignal 124. The gate of the pull-down transistor 206 is controlled bythe DLLCLK signal 118.

A second converter circuit 213 which is similar to converter circuit 211is provided for a second input (−) of comparison circuit 220 as shown inFIG. 4. The second converter circuit 213 is of similar construction tothat of converter 211, except its pull-down transistor 212 is controlledby the DLLCLK* signal 122. The upper plate of the capacitor 216 in thesecond converter circuit 213 is connected to a second input (−) of thecomparison circuit 220. Comparison circuit 220 compares the differencesbetween the output of the converter circuits 211, 213 for the DLLCLK andDLLCLK* signals 118, 122.

When the phase lock signal 124 is low, it will precharge capacitors 214and 216 to Vref. When the phase lock signal 124 goes high to activatethe gates of the enabling transistors 204, 210, the DLLCLK signal 118controls the gate of the pull-down transistor 206 to selectively permitdischarge of the capacitor 214 during the high time of the DLLCLK signal118. Also, the DLLCLK* signal 122 controls the gate of the pull-downtransistor 212 to selectively permit the discharge of the capacitor 216during the high time of the DLLCLK*. signal 122. Because the DLLCLK* andDLLCLK signals 122, 118 are inverted and non-inverted versions of thesame clock signal, the comparison circuit 220 is able to generate anerror signal 228 corresponding to the lack of symmetry in the output ofthe DLL delay elements 106.

For example, if the ratio of high tPLH to the low tPHL portion of theDLL output is 60/40, then the comparison circuit 220 may generate anerror signal 228 of appropriate polarity during the cycle which reflectsthe duration of the asymmetry, or 10% of the clock cycle in thisexample.

The error signal 228 is translated in the arbiter block 222 into twosignals, the add signal 144 and the subtract signal 146. The add andsubtract signals 144, 146 represent delay that may be added orsubtracted, respectively, with respect to the timing of the falling edgeof an output data signal 138 in order to achieve symmetry. The timing ofthe output data signal is determined in the Rise Fall CLK Generator 132(FIG. 3). An example of using the add and subtract signals 144 and 146in the Rise Fall CLK Generator 132 is illustrated in FIG. 5.

FIG. 5 shows an exemplary Rise Fall CLK Generator 132. Each of thesignals DLLR 140 and DLLF 142 are generated by passing the internal DLLclock signals DLLCLK and DLLCLK* 118 and 122, respectively, through aRise One-Shot Generator 302, 304, which generates a high pulse of shortduration when it receives a transition from low to high. The DLLR andDLLF signals 140, 142 are used to control the rising and falling of theoutput data signal 138 (FIG. 3).

As shown in FIG. 5, a Register Delay 306 is used in the DLLF data pathupstream of the DLLF Rise One-Shot Generator 304. The add and subtractsignals 144, 146 control the amount of delay added to or subtracted fromthe DLLCLK* signal 122 before the DLLF signal 142 is generated in theDLLF Rise One-Shot Generator 304. In this way, the DLLF signal 142, andhence the falling edge of the output data signal 138, can be delayed anamount necessary to make the high tPHL and low tPLH portions of the DLLoutput signal substantially equal or within an allowed tolerance of eachother. In other words, the output data signal 138 has a substantiallysymmetric duty cycle.

It should be readily understood that FIG. 5 illustrates merely oneexample of a Rise Fall CLK Generator 132. Use of the Register Delay 306in the DLLF data path is not required and it should be readilyunderstood that a different delay circuit may be used in the DLLR datapath with appropriate modifications to associated circuitry to achievethe same result. Alternatively, delay circuits may be used in both theDLLF and DLLR data paths with appropriate modifications to associatedcircuitry to achieve the same result. Also, the use of a Register Delay306 is not required and other circuit elements may be used for timingsynchronization as is well known in the art.

As demonstrated in the exemplary timing diagram of FIG. 6, by adjustingthe delay of the DLLF signal 622, the output data DQ 624 can begenerated with a 50/50 ratio (duty cycle). For example, in FIG. 6 thesystem clock XCLK 602 is shown with a 60/40 ratio of high tPLH to lowtPHL signal portions. Due to delays added by the DLL Delay Elements 106,the DLLCLK and DLLCLK* signals 604, 606 have a 65/35 ratio.

As shown in the first timing sequence 650, prior to phase lock or anycompensation using the circuitry of the invention, the DLLCLK andDLLCLK* signals 604, 606 may produce corresponding DLLR and DLLF signals608, 610, having a duty cycle not substantially equal to 50/50 and notin phase with the system clock XCLK signal 602.

The second timing sequence 670 is produced after the phase-locking iscompleted by phase detector 108, but before the operation of thecomparator 148. This second sequence 670 shows signals DLLR and DLLFsignals 616, 618 generated in phase with the rising edge of the systemclock XCLK 602, but still having the asymmetric duty cycle of the systemclock and further exacerbated by the DLL Delay Elements 106.

Finally, the third timing sequence 690 is produced using the comparator148 and accompanying adjustment of the timing of the DLLF signal 142.The subtract signal 620 is generated in the arbiter block 222 of thecomparator 148 (FIG. 4) and used to adjust the Register Delay 306 in theRise Fall CLK Generator 132 (FIG. 5), thereby adjusting the timing ofthe DLLF signal 622, as shown in FIG. 6. The resulting output datasignal 624 has an acceptable ratio of high tPLH to low tPHL signalportions and thus exhibits a substantially symmetric 50/50 duty cycle,

The symmetric quality of the output data signal 624 permits improvementof the timing budget by maximizing the data eye used for synchronizationof data output.

Thus, in reference to FIGS. 3-6, to produce a symmetric data outputsignal DQ 138 having a rising edge aligned with the rising edge of theXCLK 102, a phase detector 108, comparator 148 and Rise Fall CLKGenerator 132 are used to separately initiate rising and falling edgesof the DQ signal 138. When a system clock signal XCLK 102 is received,it is processed and compared with a signal representative of the timingof a DQ signal 138. The processed system clock signal CLKIN 103 isdelayed by DLL Delay Elements 106 controlled by a phase detector 108 toproduce a delayed system clock signal DLLCLK 118. The inverse of thedelayed system clock signal DLLCLK* 122 is then further delayed by aRegister Delay 306 under the control of a comparator 148. In this way,the rising edge of the system clock signal XCLK 102 may be aligned(phase locked) with the rising edge of the data output signal DQ 138 andthe data output signal DQ 138 may be generated so that it is symmetric.

FIG. 7 illustrates a processor system which employs logic circuits andselection methodologies in accordance with the method and apparatus ofthe invention.

As shown in FIG. 7, a processor based system, such as a computer system700, for example, generally comprises a central processing unit (CPU)702, for example, a microprocessor, that communicates with one or moreinput/output (I/O) devices 712, 714, 716 over a system bus 722. Thecomputer system 700 also includes random access memory (RAM) 718, a readonly memory (ROM) 720 and, in the case of a computer system may includeperipheral devices such as a floppy disk drive 704, a hard drive 706, adisplay 708 and a compact disk (CD) ROM drive 710 which also communicatewith the processor 702 over the bus 722. The RAM 718 is preferablyconstructed with delay-lock loop (DLL) circuitry for synchronizing thedata output of the memory devices with a system clock using the methodand apparatus of the invention described above with reference to FIGS.3-6. It should be noted that FIG. 7 is merely representative of manydifferent types of processor system architectures which may employ theinvention.

As illustrated in FIG. 8, in another embodiment of the invention, amemory system 900 is provided including at least one or a plurality ofmemory devices 933 constructed with delay-lock loop (DLL) circuitrywhich can be used to synchronize the data output of the memory devices933 with a system clock using the method and apparatus of the inventiondescribed above with reference to FIGS. 3-6. Within the memory system900, some or all of the plurality of memory devices 933 may be arrangedon at least one memory module 935. In a preferred configuration, thememory system 900 would include a plurality of memory modules 935, eachcontaining at least one or a plurality of memory devices 933 constructedwith the synchronizing circuitry as described above with reference toFIGS. 3-6.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as limited by the foregoing description but is onlylimited by the scope of the appended claims.

1-49. (canceled)
 50. A method of synchronizing an output signal with afirst clock signal, said method comprising: deriving a local clocksignal from the first clock signal; producing a delayed clock signalfrom the local clock signal; producing first and second timing signals,each associated with one of a rising and falling edge of the delayedclock signal; adjusting a relative timing of one of the first and secondtiming signals to produce substantially equal time durations betweenoccurrences of the first and second timing signals; and generating theoutput signal having a rising edge synchronized with a rising edge ofthe first clock signal in response to the first and second timingsignals.
 51. The method of claim 50, wherein the output signal isgenerated with a substantially symmetric duty cycle.
 52. The method ofclaim 50, further comprising detecting a difference in phase between thelocal clock signal derived from the first clock signal and a signalrepresentative of the output signal.
 53. The method of claim 50, furthercomprising measuring a difference between a low time of the delayedclock signal and a low time of an inverse of the delayed clock signal.54. The mod of claim 50, further comprising measuring a differencebetween a high time of the delayed clock signal and a high time of aninverse of the delayed clock signal.
 55. The method of claim 54, furthercomprising generating at least two adjustment signals from an errorsignal based on the measuring step, the adjustment signals being used toproduce at least one of the first and second timing signals.
 56. Themethod of claim 50, further comprising: delaying the delayed clocksignal by a fixed number of delays; and delaying the second timingsignal by a variable number of delays.
 57. A method of synchronizing anoutput signal with a first clock signal, said method comprising:deriving a local clock signal from the first clock signal; producing adelayed clock signal from a local clock signal; comparing a first delaycharacteristic associated with the delayed clock signal with a seconddelay characteristic associated with an inverse of the delayed clocksignal; generating a first one-shot timing signal from the delayed clocksignal and a second one-shot timing signal from an adjustably delayedsignal associated with the inverse of the delayed clock signal;adjusting the adjustably delayed signal to produce substantially equaltime durations between occurrences of the first and second one-shottiming signals; and generating the output signal from the first andsecond one-shot timing signals.
 58. The method of claim 57, wherein theoutput signal is generated having a substantially symmetric duty cycle.59. The method of claim 57, wherein the first delay characteristic is alow time of the delayed clock signal and the second delay characteristicis a high time of the inverse of the delayed clock signal.
 60. Themethod of claim 57, wherein the first delay characteristic is a hightime of the delayed clock signal and the second delay characteristic isa high time of the inverse of the delayed clock signal.
 61. A memorysystem comprising: a synchronization circuit, the synchronizationcircuit comprising: a first circuit configured to produce a first timingsignal based on phase-locking a first edge of an output signal to afirst edge of a first clock signal and to produce a second timing signalbased on a duty cycle of a delayed first clock signal; and a secondcircuit configured to use the first and second timing signals to outputthe output signal synchronized with the first clock signal and having asubstantially symmetrical duty cycle.
 62. The system of claim 61,wherein the second circuit is configured to output the output signal bygenerating a rising edge of the output signal in response to the firsttiming signal and generating a falling edge of the output signal inresponse to the second timing signal.
 63. The system of claim 61,wherein the second circuit is configured to output the output signal bygenerating a rising edge of the output signal in response to the secondtiming signal and generating a falling edge of the output signal inresponse to the first timing signal.
 64. The system of claim 61, whereinthe first circuit comprises: a comparator configured to compare a signalrepresentative of the output signal with a local clock signal derivedfrom the first clock signal; and a clock generator configured togenerate the delayed clock signal in response to a phase differencebetween the local clock signal and the signal representative of theoutput signal.
 65. The system of claim 61, wherein the output signal isa strobe signal.
 66. The system of claim 61, wherein the output signalis a data signal.